Cathode ray tube (CRT) displays, such as those commonly used in desk-top computers screens, function as a result of a scanning electron beam from an electron gun, impinging on phosphors of a relatively distant screen. The electrons increase the energy level of the phosphors. When the phosphors return to their normal energy level, they release the energy from the electrons as photons of light, which is transmitted through the glass screen of the display to the viewer.
Field emission displays seek to combine cathodoluminescent-phosphor technology with integrated circuit technology to create thin, high resolution displays wherein each pixel is activated by a plurality of electron emitters. This type of display technology is becoming increasingly important in appliances requiring lightweight portable screens. A promising technology is the use of a matrix-addressable array of cold cathode emission devices to excite phosphors on a screen.
U.S. Pat. No. 3,875,442, entitled "Display Panel" discloses a field emission device and more specifically discloses a display panel comprising a transparent gas-tight envelope, three main planar electrodes which are arranged within the gas-tight envelope parallel with each other. The three main electrodes are the emitter tip electrode, the grid electrode and the anode electrode, or cathodoluminescent panel. The cathodoluminescent panel may consist of a transparent glass plate, a transparent electrode formed on the glass plate, and a phosphor layer coated on the transparent electrode. The phosphor layer is made of, for example, zinc oxide which can be excited with low energy electrons. This structure is depicted in FIG. 1.
Field emission cathode structures are discussed in U.S. Pat. Nos. 3,665,241, 3,755,704, and 3,812,559. To produce the desired field emission, a potential source is provided with its positive terminal connected to the gate, or grid, and its negative terminal connected to the emitter electrode (cathode conductor substrate). The potential source may be variable for the purpose of controlling the electron emission current. Upon application of a potential between the electrodes, an electric field is established between the emitter tips and the low potential anode grid, thus causing electrons to be emitted from the cathode tips through the holes in the grid electrode. An array of points in registry with holes in low potential grids are adaptable to the production of cathodes subdivided into areas containing one or more tips from which areas emissions can be drawn separately by the application of the appropriate potentials thereto.
The clarity or resolution of a field emission display is a function of a number of factors including emitter tip sharpness, alignment and spacing of the gates, or grid openings, which surround the tips, pixel size, anode to cathode spacing, as well as-cathode-to-gate and cathode-to-screen to-screen voltages. These factors are also interrelated. Another factor which affects image sharpness is the coincidence of emitted electron strikes on the anode and the location of the desired phosphor pixel.
The distance (d) that the emitted electrons must travel from the baseplate to the faceplate is typically on the order of several hundred microns. The contrast and brightness of the display are optimized when the emitted electrons impinge on the phosphors located on the cathodoluminiscent screen, or faceplate, at a substantially 90 degree angle. If the electrons are not focused in some way upon the faceplate, especially in the case of a large d, then the initial electron trajectories will assume a substantially conical pattern having an apex angle of roughly 30 degree which detrimentally effects the contrast and brightness of the display. Moreover, the space-charge effect results in columbic repulsion among emitted electrons which tends to increase dispersion within the electron beam, as depicted in FIG. 1.
U.S. Pat. No. 5,186,670 which is hereby incorporated by reference, assigned to the present applicant, discloses a process for the formation of self-aligned gate and focus ring structures around the cold cathode emitter tip which function to collimate the emitted electrons so that the beam impinges on a smaller spot on the display screen and thus results in improved display contrast and clarity. In addition, U.S. Pat. No. 5,070,282 entitled, "An Electron Source of the Field Emission Type", discloses a "controlling electrode" placed downstream of the "extracting electrode".